This invention relates to echo imaging systems generally, and medical ultrasound imaging systems for imaging sections of a body to visualize internal structures, and more particularly to a phased array ultrasound imaging system having improved spatial resolution.
In recent years considerable progress has been made in the utilization of ultrasonic techniques for the exploration of the internal structure of living organisms. This technique has been used to visualize, measure, and record images of deep lying organs and physiological structures throughout the body.
The majority of diagnostic techniques using ultrasound are based on the pulse-echo method wherein short pulses of ultrasonic acoustic energy are periodically generated by a suitable piezoelectric transducer, or probe. Each short pulse of ultrasonic energy is focused into a narrow beam which is transmitted along a selected direction into the patient's body. The transmitted pulse eventually encounters interfaces between the various different structures as it propagates along its directional path within the body. When there is a characteristic acoustic impedance mismatch at an interface, a portion of the ultrasonic energy is reflected from the interface back toward the transducer. After generation of the transmit pulse, the transducer operates in a "listening" mode wherein it receives reflected energy or echoes from the body and converts them into electrical signals. The time of arrival of these echoes after the transmit pulse depends on the ranges of the interfaces encountered and the propagation velocity of the ultrasound. Also, the amplitude of the echo is indicative of the reflection properties of the interface and, accordingly, of the nature of the characteristic structures forming the interface. The information contained in the returning echoes are typically collected for a plurality of scanning directions to form an image which is displayed on a CRT as a cross-sectional image of the structures visualized.
Phased array transducers having multiple piezoelectric elements are commonly used to electronically select the direction of a transmitted pulse along scan lines forming a sector. These transducers are also electronically steered to listen for returning echoes along a particular direction, typically along the same scan line as the most recently fired transmit pulse.